You can go to some websites, create an account and have them manage your online wallets there. It also means that you can lose your money when they are hacked or compromised. Also, some websites make it very difficult to export the private keys.

Would you give your money safe key to strangers? The same applies for cryptocurrencies, the money is truly yours when only you have the keys. You can generate your own key pair, thanks to AddressGenerator

Features
AddressGenerator features

Use pure Swift and C code. No extra dependencies.
Support macOS 10.11
Generate key pair using OpenSSL. On macOS 10.13, it is LibreSSL 2.2.7 which is a fork of OpenSSL from 2014. OpenSSL seeds from /dev/urandom, which guarantees very good entropy, so your keys are truly random and secure.
No saving or networking. Your keys are generated and only you know about it.
Fast
Support many cryptocurrencies